1. Field of the Invention
The present invention relates to image and data processing systems and techniques. More specifically, the present invention relates to systems and techniques for adaptive non-uniformity compensation for focal plane arrays of infrared detectors and the like.
2. Description of the Related Art
Focal plane arrays are used in military, astronomical and other applications. For example, in military applications, focal plane arrays are often used for target acquisition and tracking. The seekers of missiles often use arrays of image detectors sensitive to energy in the visible or (more typically) infrared portion of the electromagnetic spectrum. Unfortunately, these arrays, referred to as focal plane arrays are subject to anomalies such as detector to detector nonuniformity in sensitivity, gain/offset and fixed pattern noise. While numerous techniques are known in the art for addressing detector to detector nonuniformity in sensitivity and gain/offset, fixed pattern noise continues to be problematic.
Fixed pattern noise (FPNs) are sensor fixed artifacts induced by non-uniform response of the focal plane array (FPA). The non-uniform response causes the FPA output to be spatially varying even when illuminated by a uniform source. Techniques such as stored non-uniformity correction can correct for the non-uniform array response under static conditions. But dynamic inputs, such as changing photon flux induced by dome heating, requires dynamic or adaptive non-uniformity compensation.
Accordingly, nonuniformity compensation (NUC) systems have been developed to address detector to detector nonuniformities. In particular, adaptive nonuniformity compensation systems (ADNUC) have been developed to address fixed pattern noise in focal plane arrays of image detectors. Traditional ADNUC systems use an additive feedback algorithm wherein a correction-offset term is accumulated from an error-term which is generated from the filtered output image. The correction term is then subtracted from the next input image. Therefore, depending on the feedback coefficients (the non-linear transfer functions), it takes about 10-30 image frames for the ADNUC systems to reduce the FPN from an initial high value to a low equilibrium value. While nominally effective, this system limits the response time of the system.
In addition, conventional ADNUC systems are not designed to remove temporally correlated temporal noise. Further, because FPNs are fixed on the focal plane and hence are temporally correlated, they present a more difficult false alarm problem than that presented by temporally uncorrelated temporal noise (TN). In traditional ADNUC systems, the accumulated correction-offset terms will cause fixed-pattern artifacts, which may lead to a high number of false alarms for target detection by the missile tracking system (i.e., the "tracker").
Further, traditional systems do not remove hot-dome shading effects. Hot dome shading is a heating of the missile dome due to aerodynamic friction effects. The heat on the dome creates a thermal background image which causes a filter mismatch in the tracker and thereby limits the performance of the system.
Hence, a need remains in the art for a system and technique for addressing fixed pattern noise in focal plane arrays. Specifically, there is a need for a system and technique for rapidly addressing fixed pattern noise, including temporal noise and dome shading, in focal plane arrays of infrared image detectors.